1. Field of Invention
This invention relates to radar, and more particularly to the derivation of a radar video noise/clutter clipping voltage level in an improved fashion.
2. Description of the Prior Art
A now well known innovation in search of scanning radars is the track-while-scan feature which allows pinpointing the instantaneous location of a target within a rather broad beamwidth, and to a range resolution which is within the ultimate range resolution of the radar, and then by means of "sliding window" computer processing, tracks the target while the scanning of an entire area continues. The tracking is updated once in each scan.
The radar antenna scans back and forth in azimuth, thereby sweeping a segment of the atmosphere in front of the radar, or the ground ahead of and below the radar, at a frequency of several Hz, while presenting strong signal returns on a plan position indicator (PPI) cathode ray tube radar scope. The radar transmits pulses of RF energy at a pulse repetition frequency which may be on the order of one or several KHz. As each pulse propagates outwardly, the energy is reflected off various target surfaces, and for surfaces which are suitably oriented with respect to the radar, return signals are received in the order in which the targets are contacted. Thus targets at a closer range appear sooner and targets at a further range appear later; by causing the sweep of the PPI scope to start at a zero range base for each pulse, the return signals will coincide with the position on the PPI scope which indicates its relative range. Since the antenna is scanning in azimuth, each pulse goes out in a different radial direction (a different azimuthal angle).
Because the beams of search radars are very broad, and because the pulsewidths are made large so as to transmit a large amount of energy for a maximum return signal (thereby to detect even weak targets such as small aircraft or motor vehicles) the target return signals indicate targets as being much larger than they are since return signals commence at a minimum range and extend through the length of the entire pulse thereby falsely indicating a still greater range, and return signals are received from the moment that the beam pattern first contacts the target, throughout several pulses while the beam pattern scans across the target, to the last pulse when any portion of the beam intersects the target. The track while scan radar narrows down the location of the target to one or two pulses in azimuth and to within one range gate resolution in range by determining a particular angle in azimuth and a particular range before and after which (in each case) one half of the energy (usually taken as a summation of voltages) is received by the antenna. All of this is known in the art, and is illustrated in more detail with respect to the drawing hereinafter. A track while scan radar of this type is disclosed, inter alia, in Frank U.S. Pat. No. 3,182,320, assigned to the U.S. Air Force.
To avoid the ambiguities which result from noise and clutter in track while scan radar systems known to the prior art, it is known to use a clipper to clip the noise and clutter out of the radar video prior to passage of the video to the track while scan processor so that very little noise (perhaps on the order of 10 percent) accompanies the target return signal. Since noise is substantially random in nature, it is possible to derive a reasonably good noise clipping voltage level simply by averaging the noise and target signals.
In the case of air-to-air radar systems, there is substantially no clutter in the atmosphere surrounding a solid target such as an aircraft. It is therefore very easy to discriminate the voltage summations which result from reflections off the target itself, and thereby the above process is readily implemented in a sufficiently accurate fashion. However, in use of a track while scan radar in an air to ground situation, the ground presents a substantial amount of clutter, which varies significantly within the azimuth/range window utilized for tracking, and which therefore significantly alters the right and left, and early and late voltage summations from the target itself. Since the clutter is far more significant in the return signal than is noise, and since the clutter is not random in nature, there is no real way to get an average value. In fact, it is impossible to ascertain the center of a target whenever the clutter in any one of the window segments is significantly different than the clutter in the opposite window segment.
In order to promote track while scan operation in air to ground radars, prior systems have used clipping of the radar video in attempts to sense only targets, while eliminating clutter and noise. However, to do so requires establishing a clipping level which will eliminate sufficient clutter and noise to allow the targets to stand out, while not reducing target signal strength below a useful level. One method known to the art for establishing a proper clipping level is to sample the clutter and noise in the area being searched by the radar and set the clipping level in accordance with the magnitude of the sample. In some cases, a noise and clutter level is sampled at the same range as the target but not at the target azimuth. In other cases, a noise and clutter level is sampled at maximum range. In either case, where the clutter varies significantly from the point at which it is sampled to the point at which the target is located, then either the clutter can be a significant portion of the summations and thereby render the target location inaccurate (in the case when the clutter and noise sample is very low compared to the clutter and noise surrounding the target), or may cause a clipping level which is so high as to prevent the target from passing through to the processor (in the case when the clutter is much higher in the sampling area than it is in the area surrounding the target) so the target appears to be as long in azimuth as its actual length plus nearly two beamwidths longer. Additionally, if there are multiple targets, a target with a good return signal may appear in one of the sample gates, thus establishing a very high clipping level so that the target being tracked is lost.